Production of catalyst coated membranes

ABSTRACT

A method for the production of catalyst coated membranes, especially catalyst coated membranes for use in fuel cells, includes raised relief printing a catalyst coating composition ( 11 ) onto the surface of an ion exchange membrane ( 16 ) to form at least one electrode layer.

This application is a 371 of PCT/US01/51385 filed on Oct. 26, 2001 whichclaims benefit of application Ser. No. 60/243,903 filed Oct. 27, 2000.

FIELD OF THE INVENTION

This invention relates to a method for the production of catalyst coatedmembranes for use in electrochemical cells, especially catalyst coatedmembranes for use in fuel cells.

BACKGROUND OF THE INVENTION

A variety of electrochemical cells falls within a category of cellsoften referred to as solid polymer electrolyte (“SPE”) cells. An SPEcell typically employs a membrane of a cation exchange polymer thatserves as a physical separator between the anode and cathode while alsoserving as an electrolyte. SPE cells can be operated as electrolyticcells for the production of electrochemical products or they may beoperated as fuel cells.

Fuel cells are electrochemical cells that convert reactants, namely fueland oxidant fluid streams, to generate electric power and reactionproducts. A broad range of reactants can be used in fuel cells and suchreactants may be delivered in gaseous or liquid streams. For example,the fuel stream may be substantially pure hydrogen gas, a gaseoushydrogen-containing reformate stream, or an aqueous alcohol; for examplemethanol in a direct methanol fuel cell (DMFC). The oxidant may, forexample, be substantially pure oxygen or a dilute oxygen stream such asair.

In SPE fuel cells, the solid polymer electrolyte membrane is typicallyperfluorinated sulfonic acid polymer membrane in acid form. Such fuelcells are often referred to as proton exchange membrane (“PEM”) fuelcells. The membrane is disposed between and in contact with the anodeand the cathode. Electrocatalysts in the anode and the cathode typicallyinduce the desired electrochemical reactions and may be, for example, ametal black, an alloy or a metal catalyst supported on a substrate,e.g., platinum on carbon. SPE fuel cells typically also comprise aporous, electrically conductive sheet material that is in electricalcontact with each of the electrodes, and permit diffusion of thereactants to the electrodes. In fuel cells that employ gaseousreactants, this porous, conductive sheet material is sometimes referredto as a gas diffusion layer and is suitably provided by a carbon fiberpaper or carbon cloth. An assembly including the membrane, anode andcathode, and gas diffusion layers for each electrode, is sometimesreferred to as a membrane electrode assembly (“MEA”). Bipolar plates,made of a conductive material and providing flow fields for thereactants, are placed between a number of adjacent MEAs. A number ofMEAs and bipolar plates are assembled in this manner to provide a fuelcell stack.

For the electrodes to function effectively in SPE fuel cells, effectiveelectrocatalyst sites must be provided. Effective electrocatalyst siteshave several desirable characteristics: (1) the sites are accessible tothe reactant, (2) the sites are electrically connected to the gasdiffusion layer, and (3) the sites are ionically connected to the fuelcell electrolyte. In order to improve ionic conductivity, ion exchangepolymers are often incorporated into the electrodes. In addition,incorporation of ion exchange polymer into the electrodes can also havebeneficial effects with liquid feed fuels. For example, in a directmethanol fuel cell, ion exchange polymer in the anode makes it morewettable by the liquid feed stream in order to improve access of thereactant to the electrocatalyst sites.

In electrodes for some fuel cells employing gaseous feed fuels,hydrophobic components such as polytetrafluoroethylene (“PTFE”) aretypically employed, in part, to render electrodes less wettable and toprevent “flooding”. Flooding generally refers to a situation where thepores in an electrode become filled with water formed as a reactionproduct, such that the flow of the gaseous reactant through theelectrode becomes impeded.

Essentially two approaches have been taken to form electrodes for SPEfuel cells. In one, the electrodes are formed on the gas diffusionlayers by coating electrocatalyst and dispersed particles of PTFE in asuitable liquid medium onto the gas diffusion layer, e.g., carbon fiberpaper. The carbon fiber paper with the electrodes attached and amembrane are then assembled into an MEA by pressing such that theelectrodes are in contact with the membrane. In MEA's of this type, itis difficult to establish the desired ionic contact between theelectrode and the membrane due to the lack of intimate contact. As aresult, the interfacial resistance may be higher than desired. In theother main approach for forming electrodes, electrodes are formed ontothe surface of the membrane. A membrane having electrodes so formed isoften referred to as a catalyst coated membrane (“CCM”). Employing CCMscan provide improved performance over forming electrodes on the gasdiffusion layer but CCMs are typically more difficult to manufacture.

Various manufacturing methods have been developed for manufacturingCCMs. Many of these processes have employed electrocatalyst coatingslurries containing the electrocatalyst and the ion exchange polymerand, optionally, other materials such as a PTFE dispersion. The ionexchange polymer in the membrane itself, and in the electrocatalystcoating solution could be employed in either hydrolyzed or unhydrolyzedion-exchange polymer (sulfonyl fluoride form when perfluorinatedsulfonic acid polymer is used), and in the latter case, the polymer mustbe hydrolyzed during the manufacturing process. Techniques that useunhydrolyzed polymer in the membrane, electrocatalyst composition orboth can produce excellent CCMs but are difficult to apply to commercialmanufacture because a hydrolysis step and subsequent washing steps arerequired after application of the electrode. In some techniques, a“decal” is first made by depositing the electrocatalyst coating solutionon another substrate, removing the solvent and then transferring andadhering the resulting decal to the membrane. These techniques also canproduce good results but mechanical handling and placement of decals onthe membrane are difficult to perform in high volume manufacturingoperations.

A variety of techniques have been developed for CCM manufacture whichapply an electrocatalyst coating solution containing the ion exchangepolymer in hydrolyzed form directly to membrane also in hydrolyzed form.However, the known methods again are difficult to employ in high volumemanufacturing operations. Known coating techniques such as spraying,painting, patch coating and screen printing are typically slow, cancause loss of valuable catalyst and require the application ofrelatively thick coatings. Thick coatings contain a large amount ofsolvent and cause swelling of the membrane which causes it to sag,slump, or droop, resulting in loss of dimensional control of themembrane, handling difficulties during processing, and poor electrodeformation. Attempts have been made to overcome such problems for massproduction processes. For example, in U.S. Pat. No. 6,074,692, a slurrycontaining the electrocatalyst in a liquid vehicle such as ethylene orpropylene glycol is sprayed on the membrane while the membrane is heldin a tractor clamp feed device. This patent teaches pretreating themembrane with the liquid vehicle prior to the spraying operation todecrease the swelling problems. However, processes employing suchpretreatment steps are complicated, difficult to control, and requirethe removal of large amounts of the vehicle in a drying operation. Suchdrying operations are typically slow and require either disposal orrecycling of large quantities of the vehicle to comply with applicableenvironmental requirements.

Accordingly, a process is needed which is suitable for the high volumeproduction of catalyst coated membrane and which avoids problemsassociated with prior art processes. Further, a process is needed whichis suitable for the direct application of an electrocatalyst coatingcomposition to a membrane in hydrolyzed form which avoids the swellingproblems associated with known processes and which does not requirecomplicated pre-treatment or post-treatment process steps.

BRIEF SUMMARY OF THE INVENTION

The invention provides a process for manufacturing catalyst coatedmembrane comprising: preparing an electrocatalyst coating compositioncomprising an electrocatalyst and an ion exchange polymer in a liquidmedium; and raised relief printing said catalyst coating compositiononto a first surface of an ion exchange membrane. The raised reliefprinting forms at least one electrode layer covering at least a part ofsaid surface of said membrane. Preferably, the raised relief printingtechnique employed is flexographic printing.

In a preferred process, the raised relief printing is repeated to formmultiple electrode layers covering the same part of the surface of themembrane. If desired, the process advantageously provides multipleelectrode layers which vary in composition. In addition oralternatively, the raised relief printing advantageously provides anelectrode layer with a predetermined nonuniform distribution ofelectrocatalyst across the electrode layer.

The process in accordance with the invention is extremely well-suited tohigh volume commercial manufacture of catalyst coated membrane. Raisedrelief printing provides thin, well-distributed layers of theelectrocatalyst composition and avoids problems associated with coatingtechniques which employ large quantities of solvent. The process isextremely versatile and can provide electrodes in any of a wide varietyof shapes and patterns and, if desired, can have electrocatalyst orother electrode materials which vary in amount or composition across theelectrode, through the thickness of the electrode or both.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the use of flexographic proof pressequipment to form electrodes on one side of a discrete length ofmembrane in accordance with the present invention.

FIG. 2 is a schematic view showing a continuous process in accordancewith the invention employing membrane roll stock utilizing threediscrete printing stations to form multiple electrode layers in acontinuous fashion.

DETAILED DESCRIPTION

This invention provides a process for manufacturing catalyst coatedmembranes which employs raised relief printing technology to applyelectrocatalyst containing coating compositions onto ion exchangemembranes. Of particular interest is such a printing process adapted forpreparing catalyst coated membranes for fuel cell applications.

Raised relief printing as used herein refers to processes which employany of a variety of types of pre-formed plates which have raised areaswhich define the shape or pattern to be printed on a substrate. In usein accordance with the present invention, the raised areas of the plateare contacted by and become coated with a liquid electrocatalyst coatingcomposition and then the raised areas are brought into contact with theion exchange membrane to deposit the composition onto the membrane.After drying, the shape or pattern defined by the raised areas isthereby transferred to the ion exchange membrane to form an electrodelayer. If desired, the relief printing is advantageously employed toform an electrode that is a build-up of multiple electrode layers.

In accordance with a preferred form of the present invention,flexographic printing is the raised relief printing method employed.Flexographic printing is a well known printing technique used widely forpackaging applications which employs elastomeric printing plates and isdescribed in the Kirk-Othmer's Encyclopedia of Chemical Technology, 4thedition, 1996, John Wiley and Sons, New York, N.Y., volume 20, pages62-128,especially pages 101-105. Such plates include sheet photopolymerplates, sheets made from liquid photopolymer and rubber printing plates.Especially useful are flexographic printing techniques which usephotopolymer printing plates. The most preferred relief printingtechnique employs solid-sheet photopolymer plates such as thephotopolymer flexographic printing plates sold by E.I. Du Pont deNemours and Company of Wilmington, Del. under the trademark Cyrel®.

The flexographic method offers considerable benefits in cost,changeover, speed, ease of printing on thin extensible substrates, suchas ion exchange membranes and in the variety of electrodes which can beprinted. The printed area may be of virtually any shape or design, bothregular or irregular, which can be transferred to the plate. Possibleshapes include circles, ovals, polygons, and polygon having roundedcorners. The shape may also be a pattern and may be intricate ifdesired. For example, flexography may be used to print an electrodehaving a shape that coincides with pathway of fuel and oxidant flowfields.

Multiple applications of the same or different coatings to the same areaare easily accomplished using flexographic printing. In existing uses offlexography, it is common to apply multiple colors of ink in closeregistration and these techniques are well-suited to the printing ofelectrodes having overlying multiple layers. The composition and theamount of coating applied per application may be varied. The amount ofcoating applied at each pass may be varied across the coated area, i.e.,length and/or width. Such variation need not be monotonic or continuous.The precision of flexographic printing has the further advantage ofbeing very economical in the use of coating solution, which isparticularly important for expensive electrocatalyst coatings.

The process of the invention also preferably includes the raised reliefprinting of a catalyst coating composition onto the opposite surface ofan ion exchange membrane to form at least one electrode layer coveringat least a part of the opposite surface of the membrane in registrationwith the electrode layer first applied to the membrane. Again, theability of flexographic printing to handle multiple applications inclose registration is useful for this aspect of the invention.

In the preferred flexographic printing method in accordance with theinvention using solid-sheet photopolymer flexographic plates,commercially-available plates such those sold under the trademark Cyrel®are well adapted for use in the process. Cyrel® plates are thick slabsof photopolymer uniformly deposited/bonded to 5 to 8 mil poly(ethyleneterephthalate) (PET), then capped with a thin easy-release PETcoversheet. The photopolymer itself is a miscible mixture of about 65%acrylic polymer(s), 30% acrylic monomer(s), 5% dyes, initiators, andinhibitors. U.S. Pat. Nos. 4,323,636 and 4,323,637 disclose photopolymerplates of this type.

Negatives having images to create the raised areas on the plate can bedesigned by any suitable method and the creation of negativeselectronically has been found to be especially useful. Upon UV exposurethrough the negative, monomer polymerization occurs in select areas.Following removal of the PET coversheet, unexposed, non-polymerizedmaterial may be removed by a variety of methods. The unexposed areas maybe simply washed away by the action of a spray developer. Alternatively,the non-polymerized monomer may be liquefied by heating and then removedwith an absorbent wipe material. A compressible photopolymer reliefsurface, made to photographic resolution is thus created. This reliefsurface serves to transfer electrocatalyst coating composition from abulk applicator to a print applicator or to the substrate surfaceitself. Formation of an electrode layer occurs by simple wetting coupledwith mechanical compression of the elastomeric plate.

When rubber printing plates are employed, the pattern may be generatedby known techniques including molding said rubber plate in the desiredpattern or by laser ablation to yield the desired shape or pattern.

The process of the present invention employs electrocatalyst coatingcompositions which are adapted for use in the raised relief printingprocess. The compositions include an electrocatalyst and an ion exchangepolymer in a suitable liquid medium. The ion exchange polymer performsseveral functions in the resulting electrode including serving as abinder for the catalyst and improving ionic conductivity to catalystsites. Optionally, other components are included in the composition,e.g., PTFE in dispersion form.

Electrocatalysts in the composition are selected based on the particularintended application for the CCM. Electrocatalysts suitable for use inthe present invention include one or more platinum group metal such asplatinum, ruthenium, rhodium, and iridium and electroconductive oxidesthereof, and electroconductive reduced oxides thereof. The catalyst maybe supported or unsupported. For direct methanol fuel cells, a(Pt—Ru)O_(X) electocatalyst has been found to be useful. Oneparticularly preferred catalyst composition for hydrogen fuel cells isplatinum on carbon, 60 wt % carbon, 40 wt % platinum such as thematerial with this composition obtainable from E-Tek Corporation Natick,Mass. which, when employed accordance with the procedures describedherein, provided particles in the electrode which are less than 1 μm insize.

Since the ion exchange polymer employed in the electrocatalyst coatingcomposition serves not only as binder for the electrocatalyst particlesbut also assists in securing the electrode to the membrane, it ispreferable for the ion exchange polymers in the composition to becompatible with the ion exchange polymer in the membrane. Mostpreferably, exchange polymers in the composition are the same type asthe ion exchange polymer in the membrane.

Ion exchange polymers for use in accordance with the present inventionare preferably highly fluorinated ion-exchange polymers. “Highlyfluorinated” means that at least 90% of the total number of univalentatoms in the polymer are fluorine atoms. Most preferably, the polymer isperfluorinated. It is also preferred for use in fuel cells for thepolymers to have sulfonate ion exchange groups. The term “sulfonate ionexchange groups” is intended to refer to either sulfonic acid groups orsalts of sulfonic acid groups, preferably alkali metal or ammoniumsalts. For applications where the polymer is to be used for protonexchange as in fuel cells, the sulfonic acid form of the polymer ispreferred. If the polymer in the electrocatalyst coating composition isnot in sulfonic acid form when used, a post treatment acid exchange stepwill be required to convert the polymer to acid form prior to use.

Preferably, the ion exchange polymer employed comprises a polymerbackbone with recurring side chains attached to the backbone with theside chains carrying the ion exchange groups. Possible polymers includehomopolymers or copolymers of two or more monomers. Copolymers aretypically formed from one monomer which is a nonfunctional monomer andwhich provides carbon atoms for the polymer backbone. A second monomerprovides both carbon atoms for the polymer backbone and also contributesthe side chain carrying the cation exchange group or its precursor,e.g., a sulfonyl halide group such a sulfonyl fluoride (—SO₂F), whichcan be subsequently hydrolyzed to a sulfonate ion exchange group. Forexample, copolymers of a first fluorinated vinyl monomer together with asecond fluorinated vinyl monomer having a sulfonyl fluoride group(—SO₂F) can be used. Possible first monomers include tetrafluoroethylene(TFE), hexafluoropropylene, vinyl fluoride, vinylidine fluoride,trifluoroethylene, chlorotrifluoroethylene, perfluoro (alkyl vinylether), and mixtures thereof. Possible second monomers include a varietyof fluorinated vinyl ethers with sulfonate ion exchange groups orprecursor groups which can provide the desired side chain in thepolymer. The first monomer may also have a side chain which does notinterfere with the ion exchange function of the sulfonate ion exchangegroup. Additional monomers can also be incorporated into these polymersif desired.

Especially preferred polymers for use in the present invention include ahighly fluorinated, most preferably perfluorinated, carbon backbone witha side chain represented by the formula—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₃H, wherein R_(f) and R′_(f) areindependently selected from F, Cl or a perfluorinated alkyl group having1 to 10 carbon atoms, a=0, 1 or 2. The preferred polymers include, forexample, polymers disclosed in U.S. Pat. No. 3,282,875 and in U.S. Pat.Nos. 4,358,545 and 4,940,525. One preferred polymer comprises aperfluorocarbon backbone and the side chain is represented by theformula —O—CF₂CF(CF₃)—O—CF₂CF₂SO₃H. Polymers of this type are disclosedin U.S. Pat. No. 3,282,875 and can be made by copolymerization oftetrafluoroethylene (TFE) and the perfluorinated vinyl etherCF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride) (PDMOF),followed by conversion to sulfonate groups by hydrolysis of the sulfonylfluoride groups and ion exchanging to convert to the acid, also known asthe proton form. One preferred polymer of the type disclosed in U.S.Pat. Nos. 4,358,545 and 4,940,525 has the side chain —O—CF₂CF₂SO₃H. Thispolymer can be made by copolymerization of tetrafluoroethylene (TFE) andthe perfluorinated vinyl ether CF₂═CF—O—CF₂CF₂SO₂F,perfluoro(3-oxa-4-pentenesulfonyl fluoride) (POPF), followed byhydrolysis and acid exchange.

For perfluorinated polymers of the type described above, the ionexchange capacity of a polymer can be expressed in terms of ion exchangeratio (“IXR”). Ion exchange ratio is defined as number of carbon atomsin the polymer backbone in relation to the ion exchange groups. A widerange of IXR values for the polymer are possible. Typically, however,the IXR range for perfluorinated sulfonate polymer is usually about 7 toabout 33. For perfluorinated polymers of the type described above, thecation exchange capacity of a polymer is often expressed in terms ofequivalent weight (EW). For the purposes of this application, equivalentweight (EW) is defined to be the weight of the polymer in acid formrequired to neutralize one equivalent of NaOH. In the case of asulfonate polymer where the polymer comprises a perfluorocarbon backboneand the side chain is —O—CF₂—CF(CF₃)—O—CF₂—CF₂—SO₃H (or a salt thereof,the equivalent weight range which corresponds to an IXR of about 7 toabout 33 is about 700 EW to about 2000 EW. A preferred range for IXR forthis polymer is about 8 to about 23 (750 to 1500 EW), most preferablyabout 9 to about 15 (800 to 1100 EW).

The liquid medium for the catalyst coating composition is one selectedto be compatible with the process. It is advantageous for the medium tohave a sufficiently low boiling point that rapid drying of electrodelayers is possible under the process conditions employed, providedhowever, that the composition cannot dry so fast that the compositiondries on the relief printing plate before transfer to the membrane. Whenflammable constituents are to be employed, the selection should takeinto any process risks associated with such materials, especially sincethey will be in contact with the catalyst in use. The medium should alsobe sufficiently stable in the presence of the ion exchange polymerwhich, in the acid form, has strong acidic activity. The liquid mediumtypically will be polar since it should be compatible with the ionexchange polymer in the catalyst coating composition and be able to“wet” the membrane. While it is possible for water to be used as theliquid medium, it is preferable for the medium to be selected such thatthe ion exchange polymer in the composition is “coalesced” upon dryingand not require post treatment steps such as heating to form a stableelectrode layer.

A wide variety of polar organic liquids or mixtures thereof can serve assuitable liquid media for the electrocatalyst coating composition. Waterin minor quantity may be present in the medium if it does not interferewith the printing process. Some preferred polar organic liquids have thecapability to swell the membrane in large quantity although the amountof liquids the electrocatalyst coating composition applied in accordancewith the invention is sufficiently limited that the adverse effects fromswelling during the process are minor or undetectable. It is believedthat solvents with the capability to swell the ion exchange membrane canprovide better contact and more secure application of the electrode tothe membrane. A variety of alcohols are well-suited for use as theliquid medium.

Preferred liquid media include suitable C4 to C8 alkyl alcoholsincluding, n-, iso-, sec- and tert-butyl alcohols; the isomeric 5-carbonalcohols, 1,2- and 3-pentanol, 2-methyl-1-butanol, 3-methyl, 1-butanol,etc., the isomeric 6-carbon alcohols, e.g. 1-, 2-, and 3-hexanol,2-methyl-1-pentanol, 3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl,1-pentanol, 4-methyl-1-pentanol, etc., the isomeric C7 alcohols and theisomeric C8 alcohols. Cyclic alcohols are also suitable. Preferredalcohols are n-butanol and n-hexanol. Most preferred is n-hexanol.

The amount of liquid medium in the electrocatalyst composition will varywith the type of medium employed, the constituents of the composition,the type of printing equipment employed, desired electrode thickness,process speeds etc. The amount of liquid employed is highly dependent onviscosity of the electrocatalyst composition that is very important toachieve high quality electrodes with a minimum of waste. When n-butanolis employed as the liquid medium, a coating solids content of from about9 to about 18% by weight is a particularly useful flexographic printingrange. Below about 9% solids, viscosity is undesirably low leading torapid settling of the catalytic particles, physical leaking from coatingapplicator “fountain” in standard presses and undesirably low printdeposition weights. Furthermore, at levels of n-butanol greater thanabout 91% by weight, undesirable swelling of perfluorinated sulfonicacid membranes can result. Moreover, above about 18 wt % coating solids,the electrocatalyst coating compositions takes on a paste-likeconsistency with the associated handling problems, irregular platewetting, etc.

Handling properties of the electrocatalyst coating composition, e.g.drying performance, can be modified by the inclusion of compatibleadditives such as ethylene glycol or glycerin up to 25% by weight basedon the total weight of liquid medium.

It has been found that the commercially available dispersion of the acidform of the perfluorinated sulfonic acid polymer, sold by E.I. du Pontde Nemours and Company under the trademark Nafion®, in a water/alcoholdispersion, can be used as starting material to prepare anelectrocatalyst containing coating suitable for use in flexographicprinting. The method of preparation involves the replacement of thelower alcohols and water in the commercially available dispersion with aC4 to C8 alkyl alcohol through a distillation process. The result is ahighly stable dispersion of perfluorinated sulfonic acid polymer in a C4to C8 alkyl alcohol with a water content less than 2%, more typicallyless than 0.5%. Solids content can be varied up to 20%. Using thismodified dispersion as base for the electrocatalyst coating composition,the catalytic metal or carbon black supported catalytic metal requiredto form an electrode can be added which yields a coating compositionwith excellent printing properties in the process of the presentinvention.

In the electrocatalyst coating composition, it is preferable to adjustthe amounts of electrocatalyst, ion exchange polymer and othercomponents, if present, so that the electrocatalyst is the majorcomponent by weight of the resulting electrode. Most preferably, theweight ratio of electrocatalyst to ion exchange polymer in the electrodeis about 2:1 to about 10:1.

Utilization of the electrocatalyst coating technique in accordance withthe process of the present invention can produce a wide variety ofprinted layers which can be of essentially any thickness ranging fromvery thick, e.g., 20 μm or more very thin, e.g., 1 μm or less. This fullrange of thicknesses can be produced without evidence of cracking, lossof adhesion, or other inhomogeneities. Thick layers, or complicatedmulti-layer structures, can be achieved by utilizing the very precisepattern registration available using flexographic printing technology toprovide multiple layers deposited onto the same area so that the desiredultimate thickness can be obtained. On the other hand, only a few layersor perhaps a single layer can be used to produce very thin electrodes.Typically, 1-2 μm thick layers are produced with each printing.

The multilayer structures mentioned above permit the electrocatalystcoating to vary in composition, for example the concentration ofprecious metal catalyst can vary with the distance from the membranesurface. In addition, hydrophilicity can be made to change as a functionof coating thickness, e.g., layers with varying ion exchange polymer EWcan be employed. Also, protective or abrasion-resistant top layers maybe applied in the final layer applications of the electrocatalystcoating.

Composition may also be varied over the length and width of theelectrocatalyst coated area by controlling the amount applied as afunction of the distance from the center of the application area as wellas by changes in coating applied per pass. This control is useful fordealing with the discontinuities that occur at the edges and corners ofthe fuel cell, where activity goes abruptly to zero. By varying coatingcomposition or plate image characteristics, the transition to zeroactivity can be made gradual. In addition, in liquid feed fuel cells,concentration variations from the inlet to the outlet ports can becompensated for by varying the electrocatalyst coating across the lengthand width of the membrane.

Membranes for use in accordance with the invention can be made of thesame ion exchange polymers discussed above for use in theelectrocatalyst coating compositions. The membranes can be made by knownextrusion or casting techniques and have thicknesses which can varydepending upon the application and typically have a thickness of 350 μmor less. The trend is to employ membranes that are quite thin, i.e., 50μm or less. The process in accordance with the present in invention iswell-suited for use in forming electrodes on such thin membranes wherethe problem associated with large quantities of solvent during coatingare especially pronounced. While the polymer may be in alkali metal orammonium salt form during the relief printing process, it is preferredfor the polymer in the membrane to be in acid form to avoid posttreatment acid exchange steps. Suitable perfluorinated sulfonic acidpolymer membranes in acid form are available under the trademark Nafion®by E.I. du Pont de Nemours and Company.

Reinforced perfluorinated ion exchange polymer membranes can also beutilized in CCM manufacture by the inventive printing process.Reinforced membranes can be made by impregnating porous, expanded PTFE(ePTFE) with ion exchange polymer. ePTFE is available under thetradename Goretex® from W. L. Gore and Associates, Inc., Elkton Md., andunder the tradename Tetratex® from Tetratec, Feasterville Pa.Impregnation of ePTFE with perfluorinated sulfonic acid polymer isdisclosed in U.S. Pat. Nos. 5,547,551 and 6,110,333.

While the process of the invention can be performed to make discretelengths of catalyst coated membrane with a limited number of electrodeson each side of the membrane, the invention is advantageously carriedout by performing the raised relief printing in a continuous fashionusing roll stock.

FIG. 1 shows the use of flexographic proof press equipment to formelectrodes on one side of a discrete length of membrane in accordancewith the present invention. As shown in FIG. 1, in coating station 10,the electrocatalyst coating composition 11 is picked up by the aniloxroll 12. An anilox roll is a standardized tool of the printing industryconsisting of a precision engraved cellular surfaced roll which drawsout a uniform wet ink film from the ink reservoir. The wet ink thicknessis controlled by the specific anilox cell geometry chosen. A portion ofthis ink film is transferred to a relief printing plate 13 having aplate impression 6, such as a Cyrel® flexographic printing plate,positioned on a drum 13′. A membrane 15, such as a perfluorinatedsulfonic acid polymer membrane in acid form which is available under thetrademark Nafion® from E.I. DuPont de Nemours and Company, positioned ona rotating drum 14 picks up the electrocatalyst coating composition 11from the relief printing plate 13, to form a relief image on themembrane. The dried relief image serves as an electrode on the membrane.This can be repeated the desired number of passes to produce the desiredthickness of the electrocatalyst coating composition 11. After drying,the membrane is then turned over for application to the opposite sidesof a catalyst coating composition 11, which may be different from thefirst applied catalyst coating composition, to form an second electrode.For example, an anode may be formed on one side of the membrane and acathode on the opposite side of the membrane.

FIG. 2 shows a continuous process employing rolls stock utilizing threediscrete printing stations to form multiple electrode layers in acontinuous fashion. As shown in FIG. 2, the membrane to be coated isunwound from roll 17, past the coating station 10 shown in FIG. 1 and adrying station 16. Additional coatings and drying can be accomplished asshown in coating stations 10 a to 10 n and drying stations 16 a and 16b, on to the coated and dried membrane from coating station 10. Anynumber of coating stations may be present between 10 a and 10 ndepending of the desired thickness of the electrode to be formed ordifferent coating compositions may be applied at each coating station toform different electrodes on the surfaces of the membrane. In coatingstations 10 a and 10 n respectively, electrocatalyst coatingcompositions 11 a and 11 n are picked up by the anilox rolls 12 a and 12n and transferred to relief printing plates 13 a and 13 n, positioned ona drum 13 a′ and 13 n′. The coated and dried membrane from coatingstation 10 n is then wound onto roll 18 past idler roll 19 as shown. Themembrane may then be turned over and run though the process again toproduce electrodes on the opposite sides. The electrocatalyst coatingcompositions at the three stations may be the same or different.Additional stations can be employed on line to print on the oppositeside of the membrane so that the catalyst coated membrane may becompleted in one pass.

The direct product of the process is a length of membrane with multipleelectrodes formed on it. Preferably, the product has the ion exchangepolymer in the electrodes and in the membrane in acid form, which uponcutting, is suitable for end use without necessary processing steps. Theproduct can be stored in roll form which facilitate handling and/orsubsequent processing operation. For some applications, calendering canbe employed to consolidate the electrode structure that is useful forimproving performance and this can easily be preformed on the productstored in roll form. Other treatments to improve performance are easilyperformed on the product stored in rolls form and can include acidwashing, e.g., nitric acid washing, heat treatments, etc.

For use in making membrane electrode structures, the direct CCM productof the process, after post treating if performed, is cut into the desiresize pieces and laminated to appropriate gas diffusion media by knowntechniques. The cutting operation is preferably supplied with CCM inroll form that is fed to appropriate slitting and cutting equipment toachieve high volume manufacture.

EXAMPLES Example 1

Preparation of Alcohol Dispersions of Ion Exchange Polymer(Perfluorinated Sulfonic Acid Polymer—Acid Form)

A 3 liter rotary evaporator flask is charged with 1000 g of aperfluorinated sulfonic acid polymer dispersion (Nafion®—obtained fromDuPont), comprising 5 wt % 1100 EW perfluorinated sulfonic acid polymer(PDMOF), in 50% water−50% mixed alcohol (methanol, ethanol, 2-propanolmedia)). Rotary evaporation is commenced at 60 rpm, 15 mm Hg pressure,with the evaporation flask immersed in a 25° C. H₂O bath. A dryice/acetone bath (−80° C.) is used as the overheads condenser. Afterseveral hours of slow, steady operation, 520 gms of H₂O/mixed alcoholsis removed. As the solids level increased to a nominal 10% level, anoticeable increase in viscosity. (3→20 cps) is observed. A slowapproach to this point is necessary to avoid irreversible gelation.

After a 50 gm sample of the evaporation flask residue is removed, 450 gof n-butanol is added to the evaporation flask. The clear liquid turnsan opaque, milky white. The roto-vap operation is continued under thesame conditions for several more hours until a clear liquid product (436gms) is obtained. The final measured solids content is 9.51%. Thecondensed solvent weighes 344 g, indicative of some vapor bypassing thedry ice condenser. A thin butanol layer is observed on the bulk H₂Orecovered indicating some butanol vaporization at the conditions chosen.

Repetitions of this basic procedure yield perfluorinated sulfonic acidpolymer dispersions in n-butanol with solids contents of up toapproximately 13.5% by weight without gelation. Viscosities of thedispersions obtained are typically in the range of 500 to 2000 cps.(Brookfield/20 to 24° C.). Karl Fisher determinations indicate totalresidual H₂O content ranging up to 3% in the various dispersions.

In addition to the indicated perfluorinated sulfonic acid polymerdispersion (5% solids, 1100 EW), alternate starting perfluorinated ionexchange polymer suspensions can be utilized. For example, 990 EWperfluorinated sulfonic acid polymer (PDMOF) at 18% solids in 80% mixedalcohol−20% H₂O media produces similar results. Similarily, 1100 EWperfluorinated sulfonic acid polymer (PDMOF) at 50% solids in water andnominal 800 EW perfluorinated sulfonic acid polymer (POPF) at 5% solidsin mixed water/alcohol make similar alcohol dispersions using theprocedure described above.

In place of n-butanol, other alcohols that were used successfully in theabove procedure are n- and iso-amyl alcohol (n- and iso-pentanol),cyclohexanol, n-hexanol, n-heptanol, n-octanol, glycol ethers andethylene glycol.

Example 2

Preparation of Electrocatalyst Coatings Compostions

Using the above containing perfluorinated sulfonic acid polymer/alcoholdispersions as basic component, catalyst coatings suitable forflexographic printing of CCMs for use in fuel cells are prepared asfollows:

A 13.2 wt % solids perfluorinated sulfonic acid polymer (1100 EW—PDMOF)in n-butanol dispersion, prepared as described above (28.94 g) iscombined with 77.31 g n-butanol. The resulting mixture is then cooleddown to ˜10° C., well below the 35° C. n-butanol flash point, by theaddition of dry ice. This serves to both lower the temperature and todisplace the ambient O₂ with the generated CO₂ gas, thus providing anadded margin of safety for the addition of the potentially pyrophoriccatalyst powder (platinum supported on carbon). To the cooled mixture,18.75 g of 60/40 C/Pt (E-Tek Corporation) is added slowly with vigorousstirring in order to wet out the powder instantly and to rapidlydissipate the heat of adsorption. (˜5 minutes total). Component amountsare calculated to yield a final solids content of 18.07 wt %. Thecalculated catalyst content on a dry solids basis is calculated to be83.07 wt %.

This mixture is then combined with 100 g of zirconia cylinders (0.25inch×0.25 inch diameter) grinding media in a 250 cm³ mill jar. The jaris sealed and placed on a roll mill table at ˜200 rpm at roomtemperature for 3 to 5 days. After this dispersion method the coatingcomposition is ready for testing and printing operations.

The final coating composition at nominal 18% solids has a stiff “coldcream”—like consistency that measures in the 5,000 to 20,000 cpsviscosity range by simple Brookfield methods. Simple gravimetric solidscheck give results in the 17.8 to 18.3% range. Knife coatings on heavygauge Mylar® polyester film are useful to further characterize thecoating before printing press application. A 5 mil draw knife produces aglossy black wet coating which dries (1 hr/22° C.) to a flat black, finevelvet texture, free from large particles, cracks, craters, repellenciesand streaks.

Example 3

Preparation of CCM's Utilizing Above Electocatalyst Coating Compositions

Cyrel® flexographic printing technology (DuPont Company) is used withthe above electrocatalyst coating composition to print directly on avariety of perfluorinated sulfonic acid polymer (acid form) filmsubstrates. The press used is a GMS Print Proof system as made by GMSCo. (Manchester, England).

The as received Cyrel® flexographic plate stock is photo-imaged viastrong UV exposure to a precision pattern by a photographic contactnegative “tool”. Exposed areas of thick photopolymer mixed film are UVcrosslinked. The unexposed areas are next washed away by the appropriateCyrel® developer solution. Left behind is the cross-linked, rubberyplate surface in sharp relief areas that act to transfer coatingcompositions in precise patterns and thicknesses to a moving filmsubstrate. The flexographic plate is mounted on a roll which in rotarymotion prints the composition on the moving substrate. After printingthe moving plate is re-coated by contacting a precision cellularapplicator roll. The cellular applicator in turn receives a fresh,metered coating composition supply from a stationary reservoir or“fountain”.

To utilize this GMS printing device, a cast perfluorinated sulfonic acidpolymer membrane (990 EW PDMOF), 1.5 mils thick, approximately 3″ wideand 10′ long is mounted on the print drum. The Cyrel® flexographic plateformulation PLS was imaged to produce three 50 cm² (7×7 cm) squaresaligned vertically, with each square separated by 4 cm of non-imagearea. The plate and print drum geometry is such that 5 separate plateimpressions can be achieved per single rotation of the print drumholding the membrane. In a single print drum rotation 15 singleimpressions are made. The relative speed difference between plate andprint drum is zero over the cycle eliminating scuffing, scratches etc.The plate/film gap is adjusted to achieve plate/film contact with anadditional 2 mils of plate/film compression during the initial presssetup. This is provided by adjusting the GMS anilox roll to mountedsubstrate gaps and alignment.

The anilox cell count selected was 300 lines/inch which in printersterminology gives ˜5 billion cubic microns per sq inch. This in turntranslates to a nominal 8 to 9μ wet thickness on the anilox roll. Thiswet film layer in part transfers to the plate. The plate in turntransfers part of this wet film layer thickness from the plate to themembrane substrate.

After printing, the plate surface is immediately re-coated by immediaterotational contact with the anilox roll specifically chosen for exactcoating metering to Cyrel® flexographic plate surfaces. The typicaldeposition thickness of dried coating composition to the membranesubstrate is about 0.7 to 0.9 microns with the 18% solids formulationcoating described above. To build increased catalyst layer thickness,with typically 0.7 to 0.9 micron dried increments, with fixedcoating/plate conditions, printing is repeated one or more times withapproximately +/−0.2 mm registration on the first dried layer to yieldadditional layer(s). Additional layers can be added in successiveprint/dry applications to balance potential performance versusincremental catalyst cost. Multiple prints also tend to smooth out anydeposition non-uniformity associated with the printing process. Afterthe perfluorinated sulfonic acid polymer film is printed with one ormore layers to the desired thickness/density, it may be turned over,remounted on the drum in very precise registration with the first sideprinting, and the print process repeated to form the second side of theCCM. The mis-registration observed for as many as 12 prints/side (or 24on both sides combined) is on the order of 0.2 mm.

In this way catalyst coated membranes (CCM) have been reproduciblymachine manufactured at high speed with little or no waste. Allperfluorinated sulfonic acid polymer components were used in the acidform so that there is no need for subsequent hydrolysis steps.

In addition to using cast perfluorinated sulfonic acid polymer membrane,the same catalyst printing technique can be performed on 1 and 2 milmelt extruded membrane in the acid form and on 1 and 2 milpolytetrafluorethylene (PTFE)/Nafion® composite film substrates.

Example 4

Preparation of n-hexanol Based Ion Exchange Polymer DispersionElectrocatalyst Coating Composition and CCMs

Modifications of the procedures as described in Examples 1 and 2 areused to prepare an 18% solids electrocatalyst coating composition. Themodifications consist of replacing n-butanol with n-hexanol andreplacing the 1100 equivalent weight perfluorinated sulfonic acidpolymer starting solution with 990 equivalent weight perfluorinatedsulfonic acid polymer solution.

The resulting electrocatalyst compositions are printed on a 3 inch by 10foot strip of 2 mil thick a cast perfluorinated sulfonic acid polymermembrane (1100 EW PDMOF) (acid form) by the method described in Example3 except that 5 cm by 5 cm electrocatalyst impressions are made. Theflexographic plate contains 6 of the 5×5 cm squares; 5 contacts of theplate to the above membrane film gives 30 squares. To build catalystthickness, printing is done 4 times in precise register per film side ina print/dry, print/dry series of steps. The amount of distortion causedby alcohol swelling of the film is negligible. The thickness of thefinal product was measured around the outer border of theelectrocatalyst composition squares and also at the center of thesquares (Ono-Soki Gauging system EG 225 with 1.0 micrometeraccuracy/resolution). The average substrate thickness measures 49.3micrometers. The average “substrate plus two-sided catalyst” thicknessmeasures 56.7 micrometer. By difference, the total catalyst thickness is7.4 micrometer, which is 3.7 micrometer per side. By calculation, thedried catalyst thickness per print impression is 0.93 micrometer.

Example 5

Fuel Cell Testing of CCM's

A 5×5 cm CCM prepared as in Example 4 is tested in a single cellhydrogen-air fuel cell using carbon cloth gas diffusion media sold underthe trademark ELAT® by E-Tek Corporation under the following conditions.

Experimental Conditions

-   Fuel Cell Clamping Force=4.0 ft-lb-   Fuel Cell Temperature=80° C.-   Anode Gas=Hydrogen-   Anode Gas Stoichiometry=1.5 at 2 A/cm²-   Anode Pressure=15 PSI-   Cathode Gas=Air-   Cathode Stoichiometry=2.0 at 2 A/cm²-   Cathode Pressure=15 PSI-   Anode and cathode gases were humidified.

Fuel Cell Performance Data

Cell Current Power Voltage Current Density Density (volts) (amps)amps/cm² watts/cm² 0.306 40.380 1.615 0.494 0.402 38.330 1.533 0.6160.499 34.580 1.383 0.690 0.595 28.380 1.135 0.675 0.708 15.280 0.6110.433 0.805 3.640 0.146 0.117 0.896 00.000 0.000 0.000

Example 6

The procedures described in Examples 1, 2 and 4 were used to prepare an18% solids electrocatalyst coating composition based on n-hexanol and990 equivalent weight perfluorinated sulfonic acid polymer solution withthe catalyst being 60 wt % Pt on carbon as supplied by Johnson Matheyunder the designation FC-60. The dry catalyst to polymer weight ratiowas held at 5:1. This was printed on the same film substrate as inExample 4 with the same flexographic plate on the same equipment withthe following exception: a anilox cell count used was 140 lines/inchwith nominal 10.5 billion cubic microns/square inch instead of a 300lines/inch. This provides an approximate 17 μm ink wet thickness on theanilox surface.

Film samples were taken from this process after 2, 4, 6, 8 printimpressions. The dried printed catalyst areas were analyzed underInductively Coupled Plasma (ICP) to determine platinum content/area as afunction of the number of print impressions. The results were:

Print Impressions Pt Loading (mg/cm2) 2 0.09 4 0.16 6 0.29 8 0.41

This set of data allows the following linear relationship (Y=mX+b) to beformed for this particular ink/anilox combination with 98.56% R²Correlation:Pt Loading (mg/cm2)=0.0545×(#Print Impressions)−0.035wherein Y=Pt Loading (mg/cm2); X=#Print Impressionsm (slope)=0.0545; and b (intercept)=−0.035

In this fashion, by adjusting the number of impressions, using differentanilox roll sizes, altering the platinum content of the catalystparticles, the catalyst/polymer ratio, the % solids of the compositionetc. the catalyst loading can be adjusted and controlled in a number ofdifferent ways. The catalyst can be chosen to provide the requiredquality, uniformity & productivity at the lowest overall cost.

1. A process for manufacturing a catalyst coated membrane comprising:preparing an electrocatalyst coating composition comprising anelectrocatalyst and an ion exchange polymer in a liquid medium; andraised relief printing said electrocatalyst coating composition onto afirst surface of an ion exchange membrane, said relief printing formingat least one electrode layer covering at least a part of said surface ofsaid membrane.
 2. The process of claim 1 wherein said raised reliefprinting is flexographic printing.
 3. The process of claim 1 whereinsaid raised relief printing is repeated to form multiple electrodelayers covering the same part of the surface of said membrane.
 4. Theprocess of claim 3 wherein said raised relief printing provides multipleelectrode layers which vary in composition among said multiple layers.5. The process of claim 1 wherein said raised relief printing providesan electrode layer with a predetermined nonuniform distribution ofelectrocatalyst across the electrode layer.
 6. The process of claim 1further comprising raised relief printing at least onenonelectrocatalytic coating composition to form a nonelectrocatalyticlayer over at least part of the same area of the membrane which iscovered by an electrode layer.
 7. The process of claim 6 wherein saidnonelectrocatalytic layer is a protective coating covering saidelectrode layer.
 8. The process of claim 1 further comprising raisedrelief printing said catalyst coating composition onto the surfaceopposing said first surface of an ion exchange membrane, said reliefprinting forming at least one electrode layer covering at least a partof said surface opposing said first surface of said membrane inregistration with the electrode layer on said first surface.
 9. Theprocess of claim 1 wherein said ion exchange polymer in saidelectrocatalyst coating composition and in said membrane comprise highlyfluorinated ion exchange polymer.
 10. The process of claim 1 whereinsaid ion exchange polymer in said electrocatalyst coating compositionand in said membrane comprise perfluorinated ion exchange polymer.